Co-crystals of RNA polymerase II/DNA have been recently obtained with diffraction to 5 E on rotating-anode X-ray source. In view of the significant improvement from 5 E resolution with rotating-anode X-ray to 3 E at the synchrotron source in a parallel study of the polymerase alone, the co-crystal is expected to diffract to 3 E with synchrotron beam as well. A cryo-soaking procedure has already been developed, making data collection from one crystal feasible. We now propose to collect high resolution X-ray diffraction data on polymerase II/DNA cocrystal as well as its heavy atom derivatives. The goal is to obtain high quality diffraction data for determining the crystal structure of RNA polymerase II/DNA complex.